Specific recessive alleles of the T-locus in the mouse affect either the reproductive potential (resulting in either sterility or quasi-sterility) or the transmission frequencies of males which are heterozygous or homozygous for these alleles. Data show that these phenotypic expressions can be correlated with the t-bearing spermatozoan's aberrant levels of aerobic respiration. The t-bearing spermatozoa from heterozygous males (positive/t), which are transmitted in high frequencies, have a higher level of respiration than do positive-bearing spermatozoa in the presence of specific substrates. In contrast, t-bearing spermatozoa from sterile and quasi-sterile males, either homozygous or heterozygous for t-alleles (t/t), have a lower level of respiration than positive-bearing spermatozoa in the presence of these same substrates. The proposed research is designed to establish both the mechanism by which, and the earliest stage of spermatogenesis at which these abnormal levels of respiration occur. Additional studes are proposed so that the effects of other factors (e.g., abnormal ultrastructure, numbers, motility and/or viability of t-bearing spermatozoa) on the male phenotypic expressions can be determined.